1. Field of the Invention
The present invention relates to a semiconductor laser to be used as a writing/reading light source for an optical disc device such as a DVD-ROM drive, a DVD-R drive, a DVD-RW drive, a DVD-RAM drive or the like, a writing light source for an LBP (laser beam printer) or the like, and a reading light source for a bar code scanner or the like. The present invention also relates to a method of producing the semiconductor laser above-mentioned.
2. Description of Related Arts
A writing/reading light source for an optical recording device indispensably requires a semiconductor laser capable of conducting oscillation in a short wavelength. A semiconductor laser having a composition of the AlGaInP type satisfies this requirement. To improve the recording speed, it is also required to drive a high-output laser light in a moment. A ridge-type semiconductor laser using a compound semiconductor satisfies this requirement.
FIG. 3 is a schematic section view illustrating the arrangement of a general ridge-type semiconductor laser of prior art. A lower clad layer 22 is formed on a substrate 21. An active layer 23 is formed on the lower clad layer 22. A first upper clad layer 24 is formed on the active layer 23. An etching stop layer 25 is formed on the first upper clad layer 24.
A second upper clad layer 26 and a contact layer 27 both in the form of a ridge, are laminated on the etching stop layer 25 in the form of a continuous film, thus forming a ridge portion 32. On the etching stop layer 25, block layers 30 are formed at both sides of the ridge portion 32. That is, the etching stop layer 25 is present under the second upper clad layer 26 and also under the block layers 30.
A cap layer 31 is formed on the contact layer 27 and the block layers 30.
FIG. 4(a) to FIG. 4(e) are schematic section views illustrating the steps of producing the above-mentioned general ridge-type semiconductor laser of prior art. Successively formed on the substrate 21 are the lower clad layer 22, the active layer 23, the first upper clad layer 24, the etching stop layer 25, the second upper clad layer 26 and the contact layer 27. A mask layer 28 is then formed, and a resist pattern 29 is formed on the mask layer 28 (FIG. 4(a)).
The etching stop layer 25 has a composition of the GaInP type or the like, and the active layer 23 has a composition of the AlGaInP type, the GaInP type or the like. Each of the substrate 21 and the contact layer 27 has a composition of GaAs or the like. Other semiconductor portions have a composition of the AlGaInP type or the like. The mask layer 28 is generally composed of SiO2 or SiN.
Then, the mask layer 28 is etched in the pattern of the resist 29 (FIG. 4(b)).
After the resist 29 is removed, the contact layer 27 and the second upper clad layer 26 are etched in the form of a ridge (FIG. 4(c)). Since the etching stop layer 25 is resistant to the etching medium, the layers on and lower than the etching stop layer 25 are not etched.
Then, the block layers 30 are selectively grown at both sides of the ridge portion 32 (FIG. 4(d)). After the selective growth of the block layers 30, the mask layer 28 is removed and the cap layer 31 is then grown (FIG. 4(e)).
Each of the block layers 30 and the cap layer 31 has a composition of GaAs or the like.
In the semiconductor laser having the arrangement above-mentioned, the etching stop layer 25 is present between the block layers 30 and the first upper clad layer 24. This disadvantageously relaxes the hetero-barrier between the block layers 30 and the first upper clad layer 24, thereby to increase a leak electric current from the block layers 30. This lowers the current confinement effect for the second upper clad layer 26. The oscillation efficiency of the semiconductor laser is therefore lowered.
In view of the foregoing, the etching stop layer 25 is preferably not present, in a finished product, between the block layers 30 and the first upper clad layer 24.
In this connection, it may be proposed to remove, using an etching solution such as a solution of sulfuric acid or the like, the etching stop layer 25 exposed after the second upper clad layer 26 has been etched. However, as soon as the etching stop layer 25 is removed, there is exposed the first upper clad layer 24 which is etched at a higher etching rate. Etching control in the depth direction is therefore difficult.
It is also proposed that the exposed etching stop layer 25 is removed by thermal decomposition (Japanese Laid-Open Patent Publication (KOKAI) 4-130692). In this case, however, the application of heat causes the elements (including dopants) forming the respective portions of the semiconductor laser, to mutually diffuse, thus making laser oscillation impossible.
It is a first object of the present invention to provide a semiconductor laser in which the leak electric current flowing between the block layers and the first upper clad layer is small, thus making the oscillation efficiency high, and also to provide a method of producing the semiconductor laser above-mentioned.
It is a second object of the present invention to provide a semiconductor laser having no etching stop layer between the block layers and the first upper clad layer, and also to provide a method of producing the semiconductor laser above-mentioned.
It is a third object of the present invention to provide a mass-productive semiconductor laser in which only the etching stop layer can be removed with high precision, and also to provide a method of producing the semiconductor laser above-mentioned.
A semiconductor laser according to the present invention comprises: a lower clad layer, an active layer, a first upper clad layer, and a dry-etching stop layer, all these layers being successively laminated in this order on a compound semiconductor substrate; a second upper clad layer in the form of a ridge disposed above the first upper clad layer; an etching stop layer present only between the dry-etching stop layer and the second upper clad layer; and block layers formed at sides of the second upper clad layer.
According to the present invention, the etching stop layer is not present between the block layers and the first upper clad layer. Accordingly, the current confinement effect for the second upper clad layer is well produced. Thus, the semiconductor laser having the arrangement above-mentioned has a high oscillation efficiency.
According to the arrangement above-mentioned, it is preferred that each of the dry-etching stop layer, the first upper clad layer and the second upper clad layer is a compound semiconductor of the AlGaInP type, that the etching stop layer is a compound semiconductor of the GaInP type, and that the dry-etching stop layer is higher in Al concentration than the first upper clad layer.
By making the dry-etching stop layer higher in Al concentration than the first upper clad layer, the band barrier between the dry-etching stop layer and the block layers becomes higher. This increases the current confinement effect for the ridge portion.
A semiconductor laser producing method according to the present invention comprises the steps of: successively forming, on a compound semiconductor substrate, a lower clad layer, an active layer, a first upper clad layer, an etching stop layer, and a second upper clad layer in this order; etching the second upper clad layer, into the form of a ridge, up to the etching stop layer; removing, by dry-etching, the etching stop layer exposed after the etching step above-mentioned; and selectively growing block layers at sides of the second upper clad layer in the form of a ridge.
According to the present invention, the semiconductor laser producing method may further comprise, after the first upper clad layer has been formed and before the etching stop layer is formed, the step of forming a dry-etching stop layer on the first upper clad layer.
By forming, under the etching stop layer, a dry-etching stop layer resistant to dry-etching, only the etching stop layer can selectively be removed with high precision. More specifically, by adopting dry-etching as a method of removing the etching stop layer, there can be utilized the dry-etching stop layer which improves the etching precision.
According to the producing method above-mentioned, semiconductor lasers each having no etching stop layer between the block layers and the first upper clad layer, can be mass-produced with high precision. Accordingly, the semiconductor lasers obtained by this producing method are excellent in current confinement effect for the second upper clad layer, thus providing a high oscillation efficiency.
According to the producing method above-mentioned, the temperature of the substrate is never raised to high temperature. Accordingly, the elements forming the respective portions of the semiconductor laser do not mutually diffuse. This maintains the satisfactory characteristics of the semiconductor laser.
According to the method above-mentioned, it is preferred that each of the dry-etching stop layer, the first upper clad layer and the second upper clad layer is a compound semiconductor of the AlGaInP type, that the etching stop layer is a compound semiconductor of the GaInP type, and that the dry-etching stop layer is higher in Al concentration than the first upper clad layer.
For example, when dry-etching is conducted with the use of a chlorine-type gas, a semiconductor having a composition of the AlGaInP type is resistant to such dry-etching. Accordingly, when dry-etching is conducted with a dry-etching stop layer having such a composition formed under the etching stop layer, the layers to be etched (including the etching stop layer) can selectively be removed in a good manner.